Electrostatographic marking is a well known and commonly used method of copying or printing documents. Electrostatographic marking is typically performed by exposing a light image of an original document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the original document on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner image. That toner image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a marking substrate such as a sheet of paper. The transferred toner image is then fused to the marking substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
The foregoing generally describes a typical black and white electrostatographic marking machine. Electrostatographic marking can also produce color images by repeating the above process once for each color that makes the color image. For example, the charged photoreceptor may be exposed to a light image which represents a first color, say cyan. The resultant electrostatic latent image can then be developed with cyan toner particles to produce a cyan image which is subsequently transferred to a marking substrate. The foregoing process can then be repeated for a second color, say magenta, then a third color, say yellow, and finally a fourth color, say black. Beneficially each color toner image is transferred to the marking substrate in superimposed registration so as to produce the desired composite toner powder image on the marking substrate.
Electrostatographic marking machines have increasingly utilized digital technology to produce permanent outputs from video data representations of an original image. In that case, it is beneficial to use a raster output scanner (ROS) for exposing the charged photoreceptive surface so as to record an electrostatic latent image. Generally, a raster output scanner includes a laser that generates a laser beam which is modulated in conformance with the video data representation. That modulated laser beam is then directed onto the photoreceptive surface by an optics system which usually includes a lens system for forming the laser beam into a spot on the photoreceptive surface and a rotating polygon having mirrored facets. Those facets are illuminated with the laser beam and reflect that beam across the photoreceptive surface. The rotation of the polygon causes the laser beam to scan across the photoreceptive surface in a fast scan (i.e., the scan line) direction. Meanwhile, the photoreceptive surface is relatively slowly advanced in a process direction, called the slow scan direction, which is orthogonal to the fast scan direction. In this manner, the photoreceptive surface is raster scanned by a spot produced on the photoreceptive surface by the laser beam.
With raster output scanners of the type described above it is important that the raster swept spot illuminates the photoreceptive surface with a substantially uniform intensity (when turned on). Otherwise, poor print quality will result. Generally, the raster output scanner's polygon is rotated at an essentially constant angular velocity. Raster output scanners usually employ some procedure that prevents the light from the polygon facets from varying significantly in intensity as the polygon rotates. One procedure is to over-fill the rotating facets with light: i.e., the facet illuminating beam is made larger than the individual facets, thereby simultaneously illuminating two or more facets. Over-filled raster output scanners are discussed, for example, in U.S. Pat. No. 3,995,110, and have been used in the Model 9700 electronic printing system manufactured by Xerox Corporation. An advantage of over-filling is that it allows smaller facets to be used, therefore a larger number of facets can be incorporated in a given polygon diameter. This is benefical for high speed printing. However, over-filled raster output scanners have the disadvantage of relatively low power efficiency given that a substantial part of the laser beam is lost due to the spreading of the laser beam to fill several facets. Another disadvantage of over-filled raster output scanners is that they are relatively expensive.
Another procedure that prevents the light from the polygon facets from varying significantly in intensity as the polygon rotates is to under-fill the facets. In under filled raster output scanners the laser beam which illuminates the facets is adjusted to have a smaller cross-sectional area than the cross-sectional area of the facets. In this arrangement, almost all of the laser output power is available during scanning. However, under-filled systems require larger facets and therefore the systems prints at a slower speed for a given polygon diameter and motor rotation speed.
The problems with under-filled and overfilled raster output scanners has led to the procedure of facet tracking. In facet tracking the cross-sectional area of the laser beam illuminating the facets is made less than the cross-sectional area of the facets of the and the position of the spot formed by the laser beam on each facet is adjusted to follow the illuminated facet as it rotates. In facet tracking systems the laser beam remains on the active deflecting facet for a period of time which is at least equal to the duration of time required for the spot on the photoreceptive surface to sweep across the area of the photoreceptive surface which produces a latent image. Exemplary facet tracking systems are disclosed in U.S. Pat. Nos. 3,910,657 and 4,230,394.
While facet tracking systems are beneficial for high speed and optical efficiency they tend to be relatively complex in that numerous components are required to implement the procedure.
One skilled in the art will apprecitate that the use of precision optics requires not only high quality optical elements, but also tight control in the positioning of those optics in order to obtain the very precise mechanical control required to adjust the position of the laser beam on the facet. Acoustooptic facet tracking as used in the Xerox Model 9700 uses an acoustooptic modulator, various optical elements, rf-electronics and power amplifiers to modulate the gas laser. Incorporating those elements in a system which uses a modulated laser diode is a substantial burden. High quality optical elements are also relatively expensive and require a correspondingly accurate high frequency signal generator and related electronics to produce and to maintain quality scan beam positioning. Further, such systems which incorporate feedback circuits to provide mechanical reorientation of rotating or translating mirrors generally operate too slowly to correct for motion quality errors because mirror components are relatively bulky and are difficult to move precisely and quickly.
Therefore, a new method and apparatus of implementing facet tracking would be beneficial.